I want to remove password from the login page such that only username is required for logging in. So in the registration too , i will only have username . My requirement is to create a username from the admin panel and using only the username , a user can login to their panel .

That's a terrible idea. If you did that you'd remove all of the security that's currently available so anyone that knows a valid username will be able to log into that account. There's currently 1,000's of brute-force bots out there that are trying to break systems with usernames and passwords, so take away the password security and your site will get hacked very quickly. The bots do sometimes guess usernames, but it's actually very trivial to get valid usernames in most WordPress sites, so bots will get in quickly and easily.

No hooks. The concept is so outlandish that no one even thought to provide a way to do this. You would need to develop you own login form and sign on process. The password requirement is so firmly entrenched in core code that the existing script is virtually useless except as an example of what needs to happen, such as call wp_set_auth_cookie().